The use of tabular grains in photographic industry is becoming more important for many applications. The most important reason why tabular grains are so preferred nowadays is their inherent property of having an increased ratio of surface area to volume ratio. This ratio has a positive influence on the effectiveness of the spectral sensitization which is caused by a better interaction between spectral sensitizer and silver halide grain. Moreover an enhanced spectral sensitivity resulting therefrom is deteriorated to a lesser extent by desensitization occurring when adding an increased concentration of the spectral sensitizer, which may occur to a significant extent with other types of grains. Another desired effect resulting from the shape of the tabular grain is its increased covering power which is observed after processing and which is the result of an increased surface for the same crystal volume. As a consequence the possibility of coating thinner emulsion layers with lower amounts of silver is offered.
The type of tabular grain that is used can also play an important role in a lot of different applications. If silver bromide crystals are grown at a high bromide ion excess with respect to the presence of silver ions, tabular grains are easily formed. The anisotropic growth habit that is experienced is due to the presence of usually two or three twin planes parallel to a (111) plane as published by Berriman et al. in Nature, Vol.180(1957), p.293 and J. Hamilton et al. in J. Appl. Phys., Vol.35(1964), p.414.
A first model proposed by D. Hamilton et al in J. Appl. Phys., Vol.31(1960), p.1165 assumes that all side faces are composed of (111) planes forming ridges and re-entrant grooves at the twin planes. The re-entrant grooves are sites where the nucleation of a new layer occurs more easily, thus promoting tabular growth.
In the last decade some investigators suggested that the side faces should not necessarily have to consist entirely of (111) planes. Therefor another model was proposed by G. J. Bogels et al. in Acta Cryst., Vol.A53(1997), p.84, which was based on the assumption that the side faces can be composed of (111) as well as of (100) parts and that the transition between the two occurs at the twin boundaries. The tabular growth originates from faster growth on the side plane of a (100) plane than a (111) plane as this results in the creation of a substep at the twin boundary. As the top and bottom planes which still are entirely (111) planes are growing slower, two dimensional growth is induced. Opposite to the chemical ripening of {111} tabular grain emulsions wherein huge amounts of spectral sensitizing dyes should be added before addition of chemical ripening agents and starting chemical ripening in order to provide site-directed introduction of sensitivity specks, {100} tabular grains don't require the measures mentioned hereinbefore as for the said {100} tabular grains chemical and spectral sensitization are perfectly disconnectable.
The first publications on tabular grains bounded by {100} parallel major faces were related with silver iodobromide emulsions. Bogg in U.S. Pat. No. 4,063,951 and Mignot in U.S. Pat. No. 4,386,156 were the first and most important publications. Practically all following patents like e.g. EP-A's 0 534 395; 0 569 971; 0 584 815; 0 584 644; 0 602 878; 0 616 255; 0 617 317; 0 617 320; 0 617 321; 0 617 325; 0 618 492; 0 618 493; 0 653 659 and 0 653 669; U.S. Pat. Nos. 5,024,931; 5,264,337; 5,275,930; 5,292,632; 5,310,635; 5,314,798; 5,320,938; 5,356,764; 5,601,967; and WO-Applications 94/22051 and 94/22054 are related with {100} tabular emulsion grains predominantly rich in chloride and a process for preparing them wherein the tabular grain fraction showing {100} major faces is significant. The fact that research has recently been predominantly directed to silver chloride {100} tabular grains rich in silver chloride is clearly related with lack for a stabilizer, required in the stabilization by a "habit modifying agent" of the flat crystal faces of {111} tabular grains: its absence means that no problematic interactions between said stabilizer and a chemical or spectral sensitizer occur.
Profit is further taken from the already mentioned separation of chemical and spectral sensitization mechanisms, but it is clear that optimization and reproduction of chemical and spectral sensitization highly depends on the presence of a large number of tabular {100} crystals covering a high percentage amount of the total projected area of all grains in the whole emulsion crystal population as crystals having a habit differing therefrom (as e.g. small globular crystals which may be unreproducibly present therein) may take away unreproducible amounts of chemically and spectrally sensitizing agents.
Further developments in order to provide practically useful tabular {100} silver halide emulsions having large amounts of {100} crystals or grains are thus remaining of interest.